Polymerization process

ABSTRACT

A process is described for making sterically stabilized non-aqueous dispersions of composite polymer microparticles, in which (1) monomers including at least one crosslinking monomer are polymerized in an aqueous medium at a temperature at least 10° higher than the glass transition temperature of the polymer to be formed, in the presence of a block or graft copolymer stabilizing agent, under conditions such that there is at no time present a separate monomer phase, (2) further monomers, not including any crosslinking monomer, are polymerized in the dispersion thus obtained, the presence of a separate monomer phase again being avoided, and (3) the microparticles are transferred from the resulting dispersion into a non-aqueous medium which is a solvent for the non-crosslinked polymer generated in (2). The microparticles are of value for incorporation into coating compositions the main film-forming constituent of which is compatible with the non-crosslinked component of the particles.

This invention relates to the production of crosslinked polymermicroparticles suitable for incorporation in coating compositions inorder to modify the rheological properties of the compositions and/orthe physical properties of the coatings obtained therefrom.

There have previously been described a variety of coating compositionswhich incorporate polymer microparticles that are insoluble in, butstably dispersed in, the binder. Reference may be made to British PatentApplication Nos. 17122/77, 17123/77, 17124/77, 17125/77, 30236/77,33500/77, 871/78, 20096/78 (German OLS No. 2818093, 2818094, 2818095,2818100 and 2818102, European Applications Nos. 78300095 and 78300419),as well as to British Patent Specifications Nos. 1,242,054, 1,451,948and 1,538,151, and U.S. Pat. Nos. 3,929,693, 4,025,474 and 4,115,472. Ingeneral, the microparticles are incorporated in the coating compositionsfor either or both of two reasons:

(i) in order to modify the rheological characteristics of thecomposition, which influence its behaviour on application to asubstrate;

(ii) in order to modify the mechanical or physical properties of thecoating film which is obtained after application of the composition tothe substrate.

Microparticles which have been described in the foregoing patentliterature are of both simple and composite types. The simple type ofmicroparticle is an essentially spherical particle of colloidaldimensions which is homogeneous with respect to the polymer of which itis composed; its required insolubility in the binder component of thecoating composition is achieved either by choosing as the polymer inquestion one which is inherently insoluble in the binder, or byintroducing a sufficient degree of crosslinking into a polymer whichotherwise would be soluble in the binder. The composite type ofmicroparticle has a spherical core which is of a similar nature to thesimple type of particle, but the core is associated with an outer layerof a second polymer which is not crosslinked; this second polymer isfrequently chosen so as to be compatible with the main film-formingresin of the coating composition into which the microparticles are to beincorporated.

Methods for preparing the microparticles, by polymerization ofappropriate monomers, are also of various types. Of particular interestare those procedures in which the resulting polymer is obtained directlyin particulate form as a dispersion in a liquid which is a non-solventfor the polymer. The particles so obtained can be separated from thedispersion liquid and then incorporated in the coating composition, or,in suitable cases, the dispersion itself can be blended with the otherconstituents of the coating composition. Two procedures are commonlyused for making such dispersions: (i) the non-aqueous dispersionpolymerisation technique, in which the monomer is polymerised in aninert organic liquid which dissolves the monomer but not the resultingpolymer, in the presence, dissolved in the liquid, of a polymerisationinitiator and of a polymeric stabiliser whereby the resulting disperseparticles are sterically stabilised against gross flocculation; (ii) theaqueous emulsion polymerisation technique, in which the monomer ispolymerised as an emulsion in water, in the presence of a water-solublepolymerisation initiator and of a water-soluble surface-active agent,the polymer particles in this case being stabilised against grossflocculation largely as a result of their carrying electrical charges.

The available methods do, however, suffer from certain limitations inrespect of the kind of microparticle which can be successfully made bythem. In the case where the microparticles are required for themodification of the mechanical properties of a coating, in particularwhere it is desired to reduce the brittleness of a film withoutresorting to the use of an external plasticiser, the microparticlesshould be rubbery at ambient temperatures. This in turn requires thatthe polymer of which the microparticles are composed should be derivedpredominantly from so-called "soft" monomers such as ethyl acrylate,propyl acrylate, 2-ethylhexyl acrylate, butyl methacrylate,2-ethoxyethyl methacrylate, vinyl acetate or vinyl propionate. Suchmonomers do not, however, readily lend themselves to polymerisation bynon-aqueous dispersion techniques because of the difficulty of findingan inert liquid in which they are soluble but the derived polymer isinsoluble; homopolymers or copolymers of these monomers tend to beappreciably soluble in the aliphatic hydrocarbon liquids which are thefavoured media for dispersion polymerisation of monomers such as methylmethacrylate, and in consequence it is very difficult to obtain them instable, disperse form. Furthermore, where it is desired that themicroparticle polymer should be cross-linked, non-aqueous dispersiontechniques have the further limitation that, unless special precautionsare taken, cross-linking must be effected after the main polymerisationof the monomer is complete, otherwise the utilisation of the stabiliseris inefficient and there is a risk of the particles flocculating duringthe polymerisation. In practice, this rules out the introduction ofcross-linking by the use of comonomers which are polyfunctional withrespect to the polymerisation reaction, e.g. ethylene glycoldimethacrylate, and it is necessary instead to employ comonomerscontaining mutually reactive groups through which covalent linkages canbe generated, preferably in a separate, subsequent operation.

These difficulties can be avoided by polymerising such monomers,including polyfunctional monomers if desired, by the aqueous emulsiontechnique, but a different problem then arises, namely the presence inthe dispersion of ionic species derived from the initiator and/or thesurfactant used. In order to render the particles thus obtained suitablefor incorporation in coating compositions, more especially compositionsbased on non-aqueous diluents, it is necessary to free the particles asfar as possible from these ionic species, usually by first precipitatingthe particles from the aqueous dispersion, then washing them with waterand finally drying them. It is clearly a disadvantage to have to carryout these three intermediate operations, and in any event the removal ofionic species thereby is usually incomplete.

We have now found that stable dispersions of composite cross-linkedpolymer microparticles in non-aqueous media can be produced, withoutencountering any of the difficulties or limitations discussed above, bya process wherein the mircoparticles are formed by dispersionpolymerisation of monomers in an aqueous medium, avoiding the presenceof ionic species, and are subsequently transferred to a non-aqueousmedium.

According to the present invention there is provided a process for theproduction of a stable dispersion in a non-aqueous liquid medium ofcomposite polymer microparticles having a diameter of from 0.1 to 10microns, each of which comprises a core of crosslinked polymersurrounded by and grafted to a layer of non-crosslinked polymer, theprocess comprising the steps of (1) polymerising one or moreethylenically unsaturated monomers, including at least one crosslinkingmonomer, from which the crosslinked core polymer of the microparticlesis to be derived, in an aqueous medium as hereinafter defined at atemperature at least 10° C. higher than the glass transition temperatureof the core polymer in the presence in the aqueous medium as stericstabiliser of a block or graft copolymer which contains in the moleculea polymeric component which is solvatable by the aqueous medium andanother polymeric component which is not solvatable by the aqueousmedium and is capable of becoming associated with the polymer particlesformed, the concentration of free monomer in the polymerisation mixturebeing maintained throughout this process step at a level such that at notime does the free monomer form a separate phase and the total amount ofmonomer polymerised being such that the resulting dispersion contains atleast 20% by weight of microparticles, (2) polymerising, in thedispersion obtained from step (1), one or more further ethylenicallyunsaturated monomers, not including any crosslinking monomer, from whichthe non-crosslinked polymer of the outer layer is to be derived,optionally in the presence of additional block or graft copolymerstabiliser, the concentration of such further monomer in the free statein the polymerisation mixture being maintained at a level such that atno time does that free monomer form a separate phase, and (3)transferring the polymer microparticles from the resulting dispersioninto a non-aqueous liquid medium which is a solvent for thenon-crosslinkable polymer under such conditions that the particlesbecome stably dispersed therein.

By "aqueous medium" we mean herein a mixture comprising (a) at least 30%by weight of water and (b) not more than 70% by weight of a secondconstituent which is miscible with water, the nature and proportion ofwhich are such that the mixture as a whole is capable of dissolving themonomer or monomers being polymerised to the extent of at least 3% byweight but is a non-solvent for the polymer formed. The secondconstituent may be a single substance or it may be a water-misciblemixture of two or more substances.

Preferably the aqueous medium is capable of dissolving the monomer ormonomers to the extent of at least 10% by weight.

By "glass transition temperature" (Tg) we mean the temperature at whichthe polymer which is produced in the process of the invention passesfrom the glassy state to the rubbery state, or vice versa. The Tg valuein question will normally be that of the bulk polymer as 100% material,but in a case where, as subsequently described, a plasticising substanceis deliberately added to the polymerisation mixture for the purpose ofreducing the effective Tg of the polymer, the Tg value for the purposesof the invention is that of the plasticised polymer. Even where aplasticiser for the polymer is not added as such, the "environmental" Tgof the polymer under the conditions obtaining during polymerisation maybe somewhat lower than the bulk Tg value referred to above, owing tosome plasticisation of the polymer by residual monomer or otherconstituents of the polymerisation mixture. Thus it may be possible inpractice to operate with a somewhat lower minimum polymerisationtemperature than that indicated by the bulk Tg value. However, theeffect of such fortuitous plasticisation on the Tg value is difficult topredict and, whilst it can in principle be determined by simple trialand error, it is more convenient under these conditions to choose thetemperature of polymerisation by reference to the bulk Tg value. The Tgof a bulk polymer, or of a deliberately plasticised polymer, may bedetermined for the present purposes, by experimental methods which arewell known to those skilled in the art, upon polymer of the samecomposition as that which is to be formed in the process of theinvention but obtained by some other route, for example bypolymerisation of the monomers in bulk or in solution, with subsequentaddition of plasticiser where appropriate. Alternatively, Tg values canbe calculated, from a knowledge of the monomer composition of thepolymer, by known methods.

By way of illustration, the following bulk Tg values may be quoted(ratios stated are by weight): for a 50:50 methyl methacrylate/butylacrylate copolymer, 4° C.; for a 80:20 methyl methacrylate/2-ethylhexylacrylate copolymer, 41° C.; for a homopolymer of ethyl acrylate, -22°C.; for a homopolymer of methyl methacrylate plasticised in the ratio60:40 with a neopentyl glycol/butyl alcohol adipate polyesterplasticiser, 55° C. Any of these polymer compositions can besuccessfully prepared in the form of an aqueous latex by the processs ofthe invention at the polymerisation temperatures in the range 70°-90° C.which are normally employed for the polymerisation of acrylic monomersin the presence of an azo initiator.

Ethylenically unsaturated monomers which may be used in step (1) of theprocess of the invention include in particular the acrylic monomers,that is to say acrylic acid or methacrylic acid and their alkyl esterssuch as methyl methacrylate, ethyl methacrylate, butyl methacrylate,ethyl acrylate, butyl acrylate, hexyl acrylate, n-octyl acrylate, laurylmethacrylate, 2-ethylhexyl acrylate, nonyl acrylate, lauryl acrylate,benzyl methacrylate and cetostearyl acrylate, the hydroxyalkyl esters ofthe same acids such as 2-hydroxyethyl acrylate, 2-hydroxyethylmethacrylate and 2-hydroxypropyl methacrylate, and the nitriles andamides of the same acids such as acrylonitrile, methacrylonitrile,acrylamide and methacrylamide. Other monomers which may be used, eitheralone or in admixture with these acrylic monomers, include vinylaromatic compounds such as styrene, tert-butyl styrene and vinyltoluene, vinyl esters of organic and inorganic acids such as vinylacetate, vinyl propionate and vinyl chloride. Yet other comonomers whichmay be used in conjunction with any of the foregoing monomers includedialkyl maleates, dialkyl itaconates, dialkyl methylene-malonates,isoprene and butadiene.

Preferred crosslinking monomers which may be used, in order to effectcrosslinking of the core polymer of the microparticles, are monomerswhich are polyfunctional with respect to the polymerisation reaction,including the esters of unsaturated monohydric alcohols with unsaturatedmonocarboxylic acids, such as allyl acrylate, allyl methacrylate,butenyl acrylate, butenyl methacrylate, undecenyl acrylate, undecenylmethacrylate, vinyl acrylate and vinyl methacrylate, dienes such asbutadiene and isoprene, esters of saturated monohydric alcohols withpolyunsaturated carboxylic acids such as ethyl sorbate and diethylmuconate, the diesters of saturated glycols or diols with unsaturatedmonocarboxylic acids, such as ethylene glycol diacrylate, ethyleneglycol dimethacrylate, polyfunctional aromatic compounds such asdivinylbenzene, and other doubly unsaturated compounds such as themethacrylic acid ester of ethylene glycol monodicyclopentenyl ether.

Alternatively, crosslinking may be achieved by including, in themonomers being polymerised, pairs of crosslinking monomers, each memberof the pair carrying a functional group which can enter, under theconditions of polymerisation, into a condensation-type reaction with acomplementary functional group carried by the other member of the pair.Examples of suitable pairs of monomers include glycidyl(meth)acrylate+(meth)acrylic acid and glycidyl(meth)acrylate+(meth)acrylic anhydride.

The proportion of crosslinking monomer or monomers may vary according tothe degree of crosslinking which it is desired that the core polymershould exhibit, but in general a proportion of 0.2-10%, preferably0.5-1%, based on the total core monomer weight, is suitable.

According to the monomer or monomers from which the polymer forming thecore of the composite microparticles is derived, that polymer may beeither rubbery or glassy at room temperature, that is to say, it mayhave a Tg which lies either below or above room temperature,respectively. The choice of Tg depends to some extent upon the purposefor which the composite microparticles are to be incorporated in acoating composition; in particular, where it is desired to modify themechanical or physical properties of the ultimate coating film, the corepolymer is preferably rubbery at room temperature, and more preferablyhas a Tg below -20° C. An upper limit to the Tg of the core polymer maybe set by the practicality of carrying out the polymerisation at atemperature at least 10° C. higher than the Tg of that polymer.

Preferably, the temperature of polymerisation of the monomer or monomersis at least 20° C., more preferably at least 30° C., higher than theglass transition temperature of the polymer which is formed. In general,polymerisation temperatures in the range 30°-80° C. are convenient.

Thus, in principle, the temperature at which the polymerisation iscarried out will be determined first and foremost with reference to theTg value of the polymer which it is proposed to produce in dispersion,and, having decided upon that temperature, one will then proceed tochoose an appropriate composition for the aqueous medium in which theprocess is to be conducted. In order to help the maintaining of aconstant polymerisation temperature, it is preferred to arrange that theaqueous medium can boil under reflux at that temperature, and the natureand proportion of the water-miscible second constituent of the mixturewill then be selected with this object in mind. Having regard to thefact that, for many of the monomers likely to be used in the process, aneffective polymerisation temperature will lie in the range 70°-90° C.,the second constituent of the aqueous medium, or a constituent thereof,will usually require to be a liquid of boiling point significantly lowerthan that of water.

In practice, there may be some interaction between these variables; forexample, the freedom of choice of composition of the aqueous medium tosuit a particular operating temperature may be restricted by the need tofind a water-miscible second constituent which does not have a strongsolvent action on the polymer which is formed, otherwise the aqueousmedium as a whole may not be a non-solvent for the polymer and there maybe a significant production of polymer in solution rather than indispersion. In the case where the aqueous medium contains a relativelyvolatile water-miscible liquid, the available range of formulations maybe increased by including therein a further water-soluble constituentwhich does not boil below the boiling temperature of water; such afurther constituent may be either a solid or a liquid, capable ofassisting the achievement of the necessary solvent/non-solventcharacteristics in the aqueous medium. It will be desirable, however, toretain a sufficient proportion of the lower-boiling constituent topermit refluxing of the polymerisation mixture. Another factor to beborne in mind is the desirability or otherwise of the continuous phaseof the final dispersion permanently containing materials other thanwater only. Where the water-miscible liquid constituent of the aqueousmedium is sufficiently volatile to permit refluxing at thepolymerisation temperature, that constituent can usually, if desired, bestripped off by distillation when polymerisation is complete. Incontrast, a water-miscible constituent of higher boiling point may notbe removable from the continuous phase in this way.

The use of the term "aqueous medium" herein does not imply that watershould always be the major constituent of the medium in which thepolymerisation is carried out; in many cases, the water-miscibleconstituent or constituents may predominate in the mixture. In practice,as high a proportion of water as possible is employed, consistent withthe aqueous medium being capable of dissolving the monomer beingpolymerised at least to the extent necessary to avoid the existence of aseparate monomer phase, and at the same time being a non-solvent for thepolymer produced. Evidently the degree of solvency for the monomer whichthe aqueous medium is required to possess will depend upon theconcentration of free monomer in the polymerisation mixture which it isdesired to maintain during the process, which in turn will depend uponthe rate at which it is desired that the polymerisation should proceed.In practice, water will most usually constitute 30-70% by weight of theaqueous medium.

Substances which are suitable for use as the water-miscible constituentof the aqueous medium include in particular the lower aliphaticalcohols; the preferred member of this class is methanol, but ethanol isalso very suitable. Water-methanol mixtures can be prepared havingboiling points which lie both in the optimum polymerisation temperaturerange and sufficiently above the polymer glass transition temperaturesfor the process of the invention to be carried out satisfactorily insuch mixtures with a variety of acrylic or vinyl monomers. Ethanol issomewhat less preferred than methanol because its greater effectivenessas a chain terminator in the polymerisation process may make itdifficult to obtain a disperse polymer of high molecular weight, andalso because it is a more active solvent for many polymers than ismethanol. Nevertheless, ethanol is useful where the monomer mixture tobe polymerised contains an appreciable proportion of styrene. In thecase of polymers derived from acrylic or methacrylic esters of higheralcohols, e.g. lauryl methacrylate, a suitable water-miscibleconstituent is acetonitrile.

Suitable water-miscible substances having a boiling point above that ofwater include, for example, butanol, 2-methoxyethanol, 2-ethoxyethanol,ethylene glycol, diethylene glycol and tetraethylene glycol. In general,the proportion of such substances which it is possible to use in theaqueous medium will be relatively low because they tend to be effectivesolvents for many polymers.

Although simple experimentation may sometimes be called for, theformulation of a suitable aqueous medium which meets the variousrequirements set out above does not present any serious difficulty inthe majority of cases, especially if the Tg of the polymer to be formeddoes not exceed 60° C.

Steric stabilisation of the polymer particles produced in step (1) ofthe process is achieved by the presence in the polymerisation mixture ofthe block or graft copolymer. This copolymer contains in the moleculeone type of polymeric component which is solvatable by the aqueousmedium and the chains of which consequently adopt an extendedconfiguration in that medium so as to form a steric barrier around theparticles. The copolymer also contains another type of polymericcomponent which is not solvatable by the aqueous medium and is capableof becoming associated with the polymer particles formed. This copolymermay be introduced into the polymerisation mixture in various ways.Firstly, it may be introduced as a fully pre-formed starting material,prepared in a separate operation. Secondly, it may be formed in situduring the polymerisation step (1) by introducing into thepolymerisation mixture, before polymerisation begins, a "precursor"compound comprising in its molecule a polymeric component which issolvatable by the aqueous medium and an unsaturated grouping which iscapable of copolymerising with the monomer or monomers beingpolymerised. Thirdly, it may be formed, again in situ, by introducinginto the polymerisation mixture before polymerisation begins a simplepolymer of molecular weight at least 1000 which is soluble in theaqueous medium and which contains in the molecule hydrogen atoms whichare abstractable by free radicals under the conditions ofpolymerisation, the polymer thereby becoming susceptible to grafting bysome of the monomers being polymerised.

The use of a pre-formed block or graft copolymer as stabiliser in anaqueous dispersion polymerisation process is described in detail inBritish Patent Application No. 7924873. The solvatable polymericcomponent of the copolymer is derived from a water-soluble polymer,examples of which include nonionic polymers such as the polyethyleneglycols and their monoalkyl ethers, poly(ethylene oxide)--poly(propylene oxide) copolymers containing at least 40% of ethylene oxideand their monoalkyl ethers, polyvinylpyrrolidone, polyacrylamide,polymethacrylamide and polyvinyl alcohol. Preferably the molecularweight of this component is at least 1000 and more preferably at least2000. The preferred solvatable components are those derived frompolyethylene glycols, or their monoalkyl ethers, of molecular weight inthe range 2000-4000.

The second constituent of the block or graft copolymer, which is capableof associating with the disperse microparticles, can in the simplestcase be of identical or similar chemical composition to the dispersepolymer itself, which by definition is insoluble in (and therefore notsolvated by) the aqueous medium. Such a polymeric component will have aninherent tendency to associate with the disperse polymer. However, anypolymer which satisfies the more general requirement ofnon-solvatability by the aqueous medium is suitable as the secondcomponent. Examples of second polymeric components include polymers andcopolymers derived from methyl methacrylate, ethyl acrylate, butylacrylate, styrene, tert-butylstyrene, vinyl toluene, vinyl acetate andacrylonitrile; There may also be incorporated together with one or moreof these monomers a functional monomer such as acrylic acid, methacrylicacid, 2-hydroxyethyl methacrylate and 2-hydroxyisopropyl methacrylate.

The pre-formed block or graft copolymer may range in structure fromsimple block copolymers of the AB, ABA or BAB types, where A and Brepresent the solvatable and non-solvatable components respectively,through multiple block copolymers of the ABABAB . . . types, to "comb"type graft copolymers of the structure A_(n) B, in which a plurality ofthe solvatable A components are attached at intervals to a polymerbackbone constituting the hydrophobic, associatable B component.Preferably the copolymer is of this last-mentioned "comb" type and has aslight weight excess of the solvatable components A over thenon-solvatable components B, for example in a ratio of from 1.1:1 to2:1. It is also preferred that, in this type of copolymer, the value ofn, i.e. the number of A components which are attached to each Bcomponent, should be in the range 3-10.

The molecular weight of each solvatable A component is, as alreadystated, at least 1000 and preferably at least 2000, the molecular weightof each non-solvated B component is preferably at least 1000. Moreover,it is preferred that the total molecular weight of the copolymer shouldbe at least 5000.

The block or graft copolymer may be made by any of the methods which arewell known in the art. Thus the solvatable component may be preparedfirst and then copolymerised with the appropriate monomers so as tobuild up the non-solvatable, associating component, or thenon-solvatable component may be prepared first and the solvatablecomponent then produced in situ. Alternatively, the individualcomponents can both be prepared separately and then be covalently linkedto one another through the medium of suitable mutually reactive groups.Thus, for example, in the preparation of the preferred "comb" type graftcopolymers, a water-soluble polymer suitable as the A component, such asthe mono-methyl ether of a polyethylene glycol of molecular weight atleast 2000, can be converted to the acrylic or methacrylic ester, andthis intermediate product can then be subjected to freeradical-initiated copolymerisation with other unsaturated monomers suchas styrene, ethyl acrylate, or methyl methacrylate, in order to build upan appropriate non-solvatable polymer backbone constituting the Bcomponent from which are pendant a plurality of the A component sidechains. Another suitable type of addition copolymer may be made by meansof ionic polymerisation methods, for example by preparing a "living"polystyrene block and then reacting this with ethylene oxide in order tobuild up a poly(oxyethylene) block attached thereto.

If desired, the non-solvatable component of the block or graft copolymeremployed as stabiliser may contain groupings which are capable ofreacting with the monomer or monomers which are being polymerised in theprocess of the invention. By this means, the stabiliser becomescovalently linked to the disperse polymer and the stability of thelatter towards flocculation may be enhanced. Suitable reactive groupingsinclude ethylenically unsaturated groupings which can copolymerise withthe monomer, or functional groups which can react under the conditionsof polymerisation with complementary functional groups in the monomer,e.g. epoxide groups which can react with a hydroxylic monomer such as2-hydroxyethyl methacrylate. Methods of introducing such reactivegroupings into the copolymer molecule will be apparent to those skilledin the art; for example, in the preparation of a "comb" type graftcopolymer as outlined above, the unsaturated monomers with which theintermediate acrylic or methacrylic ester of polyethylene glycol iscopolymerised may include an epoxide group-containing monomer, such asglycidyl acrylate or glycidyl methacrylate. In this way, thenon-solvatable polymer backbone of the copolymer which is built up willbe caused to contain pendant glycidyl groups. The latter may be utiliseddirectly to react with a main monomer containing a functional group,such as a hydroxyl group, during the polymerisation process of theinvention. Alternatively, the graft copolymer containing the glycidylgroups may be reacted further with an ethylenically unsaturated acid,such as acrylic acid or methacrylic acid, whereby there are introducedinto the non-solvatable component of the copolymer double bonds whichcan copolymerise with the main monomer or monomers during thepolymerisation process.

Where the block or graft copolymer stabiliser is formed in situ duringthe polymerisation step (1) of the process, according to the second modeof operation outlined above, and as described in more detail in BritishPatent Application No. 47585/78, the solvatable component of the"precursor" compound will, as in the case of the pre-formed copolymer,be derived from a water-soluble polymer. Examples of water-solublepolymers suitable or preferred for this purpose are those which havebeen referred to above in connection with the use of pre-formedcopolymer. The precursor which is introduced into the polymerisationmixture is a derivative of such a water-soluble polymer containing acopolymerisable unsaturated grouping, which in the case of thepolyethylene glycols, or their ethers, may conveniently be an ester ofone of these substances with a copolymerisable unsaturated acid, forexample methacrylic acid, itaconic acid or maleic acid. Esterificationof the glycol, or ether thereof, may be effected by an ester-interchangereaction with a lower alkyl ester of the unsaturated acid, for examplewith methyl methacrylate; alternatively the glycol or its ether may bereacted with a suitable acid chloride, for example methacrylyl chloride,in the presence of a hydrogen chloride acceptor. Yet again, the glycolor its ether may be reacted directly with the unsaturated acid to givethe ester, or with its anhydride to form a half-ester. Other suitableprecursors may be obtained by reacting a carboxyl group-terminatedpolyvinylpyrrolidone (see, British Specification No. 1,096,912) withglycidyl methacrylate. Yet other suitable precursors may be obtained bythe procedure described in co-pending British Patent Application No.47584/78, that is to say by reacting a water-soluble polyalkylene glycolor its monoalkyl ether with a cyclic aliphatic carboxylic anhydride andthen reacting the resulting half-ester with an epoxy compound containinga polymerisable double bond. For example, the monoethyl ether of apolyethylene glycol is reacted with succinic anhydride and the productthen condensed with glycidyl methacrylate to give a precursor containinga terminal vinyl grouping. As explained in the co-pending Applicationreferred to, this method of making a precursor is convenient inparticular because it avoids the necessity of removing any by-productsor excess reagents, which could interfere with the subsequent use of theprecursor, that arises with most of the other methods discussed above.

Where the block or graft copolymer stabiliser is formed in situaccording to the third mode of operation outlined above, as described indetail in British Patent Application No. 7921091, the simple polymerwhich is introduced into the polymerisation mixture is a water-solublepolymer of molecular weight at least 1000 which may be either linear orbranched and either homopolymeric or copolymeric in nature, such thatall polymeric components of the molecule are soluble in the aqueousmedium (in this respect it is distinguished, when a copolymer, from thepre-formed block or graft copolymers used according to the first mode ofoperation, where by definition certain polymeric components of themolecule are not per se soluble in the aqueous medium). Furthermore, thesimple polymer does not contain any deliberately introduced double bondswhich can copolymerise with the monomer or monomers being polymerised(it is thus distinguished from the copolymerisable "precursors"previously discussed). The simple polymer is required, as stated, tocontain hydrogen atoms which are abstractable by free radicals under theconditions of polymerisation. This requirement is in practice met by allpolymers which are water-soluble, and accordingly suitable polymersinclude those which have been referred to above in connection with thesolvatable components of pre-formed copolymer stabilisers. However, itmay be an advantage if the polymer molecule contains deliberatelyintroduced groups which are especially susceptible to abstraction ofhydrogen by a neighbouring free radical. Such groups include mercapto,sec-butyl, cyanomethyl and (CH₃)₂ NCH₂ --groups, and examples ofsuitable water-soluble polymers containing them include copolymers ofvinylpyrrolidone with minor proportions of dimethylaminoethylmethacrylate, sec-butyl methacrylate or vinyl cyanoacetate.

The proportion of the block or graft copolymer stabiliser which isrequired to be present during the polymerisation process of step (1)will vary to some extent according to the particular disperse polymerwhich is concerned and the particle size which it is desired that theresulting dispersion should have. The optimum proportion in anyindividual case can readily be found by simple experiment, whether thecopolymer be pre-formed and added as such or formed in situ from eithera copolymerisable precursor or a water-soluble simple polymer. Whicheverof these three modes of operation is adopted, however, it may be statedfor general guidance that the proportion of material added (i.e.pre-formed copolymer, precursor or water-soluble polymer) will usuallylie in the range 0.5-20%, and more especially 2-10%, by weight of thedisperse polymer content of the dispersion being made.

Step (1) of the process of the invention will usually require thepresence in the polymerisation mixture of a suitable catalyst orinitiator capable of producing free radicals. Suitable substances forthis purpose are those catalysts or initiators well known for use in thepolymerisation of acrylic or vinyl monomers which are soluble in themonomers, in particular azo compounds such as azodiisobutyronitrile and4,4-azobis(4-cyanovaleric acid), or peroxy compounds such as benzoylperoxide, lauroyl peroxide and diisopropyldicarbonate. To some extent,the choice of initiator can influence the temperature at which thepolymerisation is carried out and thus may constitute a further factorto be considered in deciding the overall composition of thepolymerisation mixture as discussed above. Furthermore, the type ofinitiator used may vary according to the manner in which the block orgraft copolymer stabiliser is to be introduced into the polymerisationmixture. Where the copolymer is pre-formed and is introduced as such,azo compounds are generally to be preferred because of their reducedtendency, as compared with peroxy compounds, to promote random graftingof the monomers being polymerised on to the block or graft copolymer;such grafting could lead to decreased efficiency of the copolymer as asteric stabiliser for the disperse polymer formed. The same preferenceapplies in the case of in situ production of the copolymer via acopolymerisable precursor, since here the deliberately introducedunsaturated grouping in the precursor is designed to be the exclusivegrafting point and additional, random, grafting could again lead todifficulties. In contrast, where the copolymer is to be produced in situby hydrogen abstraction grafting on to a water-soluble simple polymer,the preference is for peroxy compounds as initiators because of theirknown ability to promote grafting by this mechanism.

Whichever type of catalyst or initiator is used, the amount requiredwill normally lie in the range 0.1-2.0%, preferably 0.5%-2%, of theweight of monomer, and the addition of this ingredient is preferablymade along with the monomers being polymerised.

There may also be present during the polymerisation process, wherenecessary, a chain transfer agent which, unlike the catalyst orinitiator, is insoluble in the aqueous medium. Examples of suitableagents are n-octylmercaptan and tert-dodecyl mercaptan. The chaintransfer agent may be used in an amount of from 0.1-2% of the weight ofmonomer. The effect of the chain transfer agent is to regulate themolecular weight of the disperse polymer and ultimately to reduce theproportion of finer particles in the disperse phase, thus increasing theaverage particle size. Such measures to regulate molecular weight arenot, however, usually employed as the water-miscible constituent of theaqueous medium. In particular, it is preferred not to use an additionalchain transfer agent in the case of the in situ production of thecopolymer stabiliser by hydrogen abstraction grafting on to awater-soluble simple polymer.

In carrying out step (1) of the process of the invention, it ispreferred to introduce the monomer or monomers gradually into theaqueous medium, rather than to add the total monomer charge all at once.This procedure may in fact be essential in many cases if the conditionis to be satisfied that at no time during the polymerisation shouldthere be present a separate monomer phase. Where two or more monomersare involved, these may be pre-mixed before being fed into the aqueousmedium. A particularly preferred procedure, whereby improved control ofparticle size of the disperse polymer is achieved, is to add initiallyto the aqueous medium a small portion of the total monomer charge,together with an appropriate amount of initiator. This initial charge,which may be added all at once provided that the aqueous medium iscapable of dissolving it completely, is allowed to polymerise first; thereaction mixture is initially clear and homogeneous, but subsequentlybecomes opalescent as a very fine "seed" dispersion of polymer isformed. Following this, the main portion of the monomer charge,containing initiator, is fed in steadily at a rate sufficient tomaintain an acceptable speed of polymerisation but not such as to causemonomer to form a separate phase in the polymerising mixture. Where thepolymerisation is carried out at the reflux temperature of the aqueousmedium it is preferred to arrange for this main monomer feed to mix withthe returning distillate so that it is well diluted before it enters thereaction zone; this distillate will normally be rich in the second,water-miscible constituent of the aqueous medium and will be a goodsolvent for the monomer being introduced. The rate of monomer feed ispreferably such that the monomer is diluted with at least its own volumeof returning distillate.

The manner of introduction of the preformed copolymer stabiliser, thecopolymerisable precursor or the graftable single polymer, as the casemay be, into the polymerisation mixture may vary somewhat according tocircumstances. In every case, some portion of the ingredient in questionis added prior to the commencement of polymerisation; thus, where apreliminary "seed" stage is operated, at least part of the total chargeof either a pre-formed copolymer or a precursor is added along with theinitial charge of monomer and the remainder of the copolymer orprecursor is then introduced in the subsequent feed of the main part ofthe monomer charge. If the seed stage procedure is used in the case of asimple polymer which is to undergo grafting, the whole of the requisiteamount of that material should be introduced with the initial monomercharge.

Where step (1) of the process is carried out using a simple polymergraftable by hydrogen abstraction it may assist in achieving a highdegree of grafting of this material if it is pre-activated before it isintroduced into the polymerisation mixture. This may be done by heatingit, preferably dissolved in some of the aqueous medium to be usedsubsequently, together with the polymerisation initiator at atemperature in the range 65° to 120° C. for a period of from 5 minutesto 1 hour; the conditions chosen should, of course, be such as not tocause the soluble polymer to undergo degradation, cross-linking or otherdeleterious changes.

In the case where the process of the invention is performed, asdescribed above, by gradual "feed" of monomer to a preformed "seed"dispersion of polymer, it is possible to form the "seed" particles frommonomer different from that which is subsequently introduced in the"feed" stage. Such "seed" monomer does not need to satisfy therequirement hereinbefore stated that the polymerisation temperatureshould be at least 10° C. higher than the glass transition temperatureof the polymer (viz. the "seed" polymer) which is formed. Thus,essentially any monomer may be used in the "seed" stage so long as itdoes not form a separate phase in the reaction mixture and so long as itgives rise to a polymer which is insoluble in the aqueous medium. Forexample, where the main disperse polymer is to be derived from a mixtureof methyl methacrylate and 2-ethylhexyl acrylate (Tg of polymer,approximately -10° C.; polymerisation temperature, 76-80° C.), it ispossible to employ methyl methacrylate alone (Tg of polymer, 105° C.) ina "seed" stage; the main monomers are then introduced in the "feed"stage to give rise to the main disperse polymer. It is, however, to beunderstood that, in a "seed-feed" procedure as just described, the"feed" stage must always be conducted in accordance with the definitionof the process of the invention hereinabove given.

Other substances which may be added to the polymerisation mixture instep (1) include, as already mentioned, a plasticiser for the dispersepolymer, where it is desired that the latter should be softer than theunmodified polymer. The addition of plasticiser may, indeed, render itpossible to apply this step of the process of the invention to certainmonomers where it would otherwise fail. For example, the homopolymer ofmethyl methacrylate has a Tg of 105° C. and it is practically impossibleto operate the present process with methyl methacrylate as the solemonomer so as to produce a stable latex; however, by the addition ofplasticiser the Tg can be brought down to a level where the process cansuccessfully be carried out. Suitable plasticisers are any of thosewhich are well known in the art for use with the particular polymer inquestion; they may be either soluble or insoluble in the aqueous medium.Conveniently the plasticiser may be added to the polymerisation mixturealong with the monomer or monomers.

The product of step (1) of the process of the invention is an aqueousdispersion of microparticle cores, which may have disperse phasecontents in the range 40-60% by weight and even as high as 70% byweight. The particles are sterically stabilised against grossflocculation. In step (2) of the process, further ethylenicallyunsaturated monomer is polymerised in the presence of this dispersion,so as to give rise to the non-crosslinked polymer with which theparticle core is associated. Suitable ethylenically unsaturated monomersfor use in this step include those referred to in connection with step(1), except that in step (2) no monomers are employed which arepolyfunctional with respect to the polymerisation reaction. The furtherpolymerisation is carried out most conveniently by following up step(1), once the conversion of monomer and crosslinking reaction in thatstep are complete, with the direct feeding into the dispersion of coreparticles thus obtained of the further monomer, if desired with theaddition of further block or graft copolymer stabiliser or of furthercopolymerisable precursor or soluble polymer, as appropriate, in orderto maintain dispersion stability of the particles during this procedure.Whilst it is necessary, as already stated, that during step (2) theformation of a separate free monomer phase is avoided, it is notnecessary that the temperature at which the further monomer ispolymerised should be at least 10° higher than the Tg of thenon-crosslinked polymer which is produced, but it is preferred that thefurther monomer does not amount to more than 50% of the total weight ofmonomer used in steps (1) and (2) combined.

As already stated, the non-crosslinked polymer must be per se soluble inthe non-aqueous medium, and the monomer or monomers from which it is tobe formed in step (2) will be chosen accordingly, following principleswell understood by those skilled in the art. However, because of thisinherent solubility in the non-aqueous medium it is necessary to ensurethat the non-crosslinked polymer is firmly attached to the cores of themicroparticles; otherwise, on carrying out step (3) of the process, thenon-crosslinked polymer may be dissolved right away from themicroparticles and the latter would lose their stability. The requiredbonding is achieved through grafting of the non-crosslinked polymer onto the microparticle core; this grafting may take place throughcopolymerisation of residual unsaturated groupings in the core polymer,originating from the crosslinking monomer used in step (1), with themonomer from which the non-crosslinked polymer is derived; someproportion of that crosslinking monomer, sufficient for this purpose,always remains incompletely polymerised or reacted in the core polymer.Alternatively grafting may occur through the medium of abstraction ofhydrogen atoms from the core polymer by free radicals present in thepolymerisation mixture, giving active centres on to which thenon-crosslinked polymer chains may grow.

Although the monomers used in forming the non-crosslinked polymer are bydefinition such that no crosslinking takes place during the formation ofthat polymer, nevertheless one or more of them may carry, in addition tothe polymerisable group, a chemically functional group whereby thenon-crosslinked polymer is rendered crosslinkable by an externalcrosslinking agent and can be thus crosslinked after the application toa substrate of a coating composition into which the microparticles areincorporated. Thus, for example, the monomers may include one carrying ahydroxyl group or a carboxyl group, which can subsequently be reactedwith a melamine-formaldehyde resin.

In carrying out step (2) of the process, there may be introduced, alongwith the monomer from which the non-crosslinked polymer is to bederived, further catalyst or initiator for the polymerisation. In theevent that it is desired to promote the grafting on of thenon-crosslinked polymer to the core polymer by the hydrogen abstractionmechanism mentioned above, the use of a peroxide catalyst may bebeneficial here.

The composite polymer microparticles which result from step (2) of theprocess are usually spherical and have diameters in the range 0.01 to 10microns, more especially in the range 0.1 to 1 micron.

In the final step of the process, the polymer microparticles, associatedwith the non-crosslinked polymer, which are obtained in dispersion inthe aqueous medium on completion of step (2), are transferred to thechosen non-aqueous medium. This is accomplished by blending the aqueousdispersion with the non-aqueous medium and then removing the water andthe water-miscible second constituent of the aqueous medium bydistillation. A particularly convenient procedure is available in thecase where the nonaqueous medium forms an azeotrope with water and withthe second constituent, namely to add the aqueous dispersion from step(2) gradually to the non-aqueous medium maintained at its boiling point,so that the water and second constituent are rapidly removed as anazeotropic distillate. There is thus finally obtained a stabledispersion of the microparticles in the non-aqueous liquid alone, thesolids concentration of which can then be adjusted as desired either byevaporation or by addition of further liquid. Where the non-aqueousliquid does not form an azeotrope with either the water or the secondconstituent, the latter materials can be removed by straightforwarddistillation provided that the non-aqueous liquid has a boiling pointhigher than that of either of them. If desired, the distillation may becarried out under sub-atmospheric pressure.

The non-aqueous liquid must, as already stated, be a solvent for thenon-crosslinked polymer. It may be either miscible or immiscible withwater. It may be of various types, according to the nature of thenon-crosslinked polymer, and may be either a single liquid or a mixture.Of particular interest are hydrocarbon liquids, both aliphatic andaromatic, such as aliphatic petroleum fractions boiling at temperatureswithin the range 80° to 260° C., cyclohexane, toluene and xylene. Othersuitable non-aqueous liquids include butanol, 2-ethoxyethanol, methylethyl ketone and methyl isobutyl ketone.

The non-aqueous dispersions of composite polymer microparticles obtainedby the foregoing procedure may be incorporated directly into a coatingcomposition, the main film-forming constituent of which is compatiblewith the non-crosslinked polymer component of the microparticles. Thiscondition will usually be satisfied if the composition in question is asolution of the film-forming material in the same non-aqueous liquid asthat of the microparticle dispersion, or in a liquid of similar polarcharacter. Alternatively, the composite microparticles may first beisolated from the dispersion, for example by spray drying, and then beincorporated into a liquid coating composition as a dry powder. As afurther alternative, the dry microparticles thus isolated may be blendedinto a powder coating composition.

According to the nature of the polymer microparticles, as alreadyindicated, their presence in the coating composition may either modifythe rheological properties of the composition so as to secureimprovements in the application characteristics of the latter, e.g. areduced tendency for "sagging" of the film to occur on the substrateimmediately following application; or it may modify the ultimateproperties of the coating film, as for example by rendering it lessbrittle.

The invention is illustrated but not limited by the following Examplesin which parts are by weight:

EXAMPLE 1 (1) Preparation of sterically stabilised latex of crosslinkedcore particles

A reaction flask was fitted with thermometer, stirrer, provision forblanketing the contents with nitrogen and an up-and-over condensersystem reconnected to the flask via a mixing chamber. The flask washeated in a water-bath. Monomer to be polymerised was fed by means of apump at a controlled rate into the mixing chamber where, under operatingconditions, it became diluted with returning distillate before enteringthe flask.

The following charges were prepared:

    ______________________________________                                        (A)    Distilled water     22.3     parts                                            Methanol            35.25    parts                                            Methacrylic acid ester of                                                     methoxypolyethylene glycol,                                                   mol. wt. 1900       1.3      part                                      (B)    Butyl acrylate      2.9      parts                                            Styrene             0.6      part                                             Azodiisobutyronitrile                                                                             0.1      part                                      (C)    Allyl methacrylate  0.6      part                                             Methacrylic acid ester of                                                     methoxypolyethylene glycol,                                                   mol. wt. 1900       1.0      part                                             Butyl acrylate      20.9     parts                                            Styrene             4.8      parts                                            Azodiisobutyronitrile                                                                             0.4      part                                      ______________________________________                                    

Charge A was introduced into the flask, Charge B was added thereto andthe mixture heated to reflux temperature (about 74° C.). After 1 hour afine bluish-white dispersion of seed polymer particles had formed, andCharge C was then fed in via the pump over a period of 3 hours. When theaddition was complete, refluxing was continued for a further 1 hour toensure complete conversion of monomers and crosslinking of the corepolymer.

(2) Modification of core particles with non-crosslinked polymer

To the dispersion of core polymer obtained from step (1), there was thenfed in via the pump, at the same temperature as before, the followingcharge:

    ______________________________________                                        (D)     Methyl methacrylate                                                                             9.7      parts                                              Azodiisobutyronitrile                                                                           0.1      part                                       ______________________________________                                    

This charge was added over a period of 1 hour and the polymerisationmixture was then held at reflux temperature for a further 1 hour withthe final addition of 0.05 part more of azodiisobutyronitrile. Themixture was thereafter allowed to cool, with stirring, to roomtemperature. There was obtained a stable dispersion of compositemicroparticles of substantially submicron size, of which the corepolymer had the composition butyl acrylate 79.9%, styrene 18.1%, allylmethacrylate 2.0%, and a theoretical Tg of -26° C. The gel content ofthe microparticles was measured by treating the particles withtetrahydrofuran, then centrifuging off, drying and weighing the residualgel. A gel content of 69.5% was found. The overall composition of themicroparticles was butyl acrylate 60.25%, styrene 13.75%, methylmethacrylate 24.5%, allyl methacrylate 1.5%. The molecular weight of thenon-crosslinked polymer was estimated by gel permeation chromatographyto be 53,000.

(3) Transfer of microparticles to non-aqueous medium

The microparticle latex obtained from step (2) (135 g) was added slowlyto refluxing toluene (300 g) in a 700-ml flask fitted with stirrer,thermometer, dropping funnel and Dean and Stark separator. The water andmethanol were removed from the system as azeotropes along with some ofthe toluene as the distillate temperature generally climbed back to 110°C. The flask then contained a clear dispersion of the microparticles intoluene; this had a solids content of 16.1% and a viscosity ofapproximately 4 poises.

EXAMPLE 2

This Example illustrates the transfer of composite polymermicroparticles, prepared as described in Example 1(1) and (2), into adifferent non-aqueous medium.

The aqueous microparticle latex resulting from the procedure of steps(1) and (2) of Example 1 (1660 g) was added over a period of about 4hours to refluxing methyl isobutyl ketone (3500 g) contained in a10-litre flask fitted as described in step (3) of Example 1. When theaddition was complete, there was begun the gradual addition of furthermethyl isobutyl ketone (4500 g). During these additions, which occupieda total period of 2 hours, a distillate mixture of methanol, water andmethyl isobutyl ketone (total amount, 2788 g) was removed. The residuein the flask was a creamy coloured dispersion of the compositemicroparticles in methyl isobutyl ketone, having a solids content of 11%and a viscosity of 0.5 poise. In contrast to the product of stage (3) ofExample 1, this dispersion was not clear because of the difference ofrefractive index between the gel polymer and the methyl isobutyl ketone.

EXAMPLE 3

A latex of modified acrylic polymer microparticles was preparedaccording to the method described in Example 1(1) and (2), but themonomer composition of the core polymer was as follows: butyl acrylate39%, styrene 10%, 2-ethylhexylacrylate 25%, acrylonitrile 24.5%, allylmethacrylate 1.5%. The non-crosslinked polymer was homopolymer methylmethacrylate and the weight ratio of core polymer to non-crosslinkedpolymer was 75:25. The core polymer had a refractive index of 1.49 and atheoretical Tg of approximately -31° C. The latex finally obtained had asolids content of 35% and the particle size was about 0.5 micron.

The modified microparticles were then transferred from the aqueous latexto toluene, following the method described in Example 1(3), using 150 gof latex and 500 g of toluene. In total, 285 g of distillate wasremoved. The resulting dispersion of the microparticles in toluene had asolids content of 15.1% and a viscosity of approximately 0.3 poise.

EXAMPLE 4

A latex of modified acrylic polymer microparticles was preparedaccording to the method of Example 1(1) and (2), with the same corepolymer composition as that described in Example 1 but with anon-crosslinked polymer derived from 2-ethylhexyl acrylate. The latexobtained had a solids content of 35%.

The modified microparticles were then transferred from the aqueous latexto white spirit, following the method of Example 1(3), using 150 g oflatex and 500 g of white spirit (boiling range 160°-200° C.).Approximately 150 g of distillate was removed, to provide a dispersionof the microparticles in the white spirit of solids content 11.5%.

EXAMPLE 5

The procedure of Example 1(1) and (2) was repeated, except that theallyl methacrylate was replaced by an equal weight of ethylene glycoldimethacrylate. The resulting modified latex, after filtration to removesome coarse material, had a particle size of 0.57 micron and a solidscontent of 34.7%.

The modified latex (100 parts) was then transferred to toluene (302parts) as described in Example 1(3), methanol and water being removeduntil a distillation temperature of 110° C. was attained. The resultingdispersion of microparticles in toluene had a viscosity of 0.9 poise andsolids content of 18.6%.

EXAMPLE 6

The procedure of Example 1(1) and (2) was repeated, but replacing themethacrylic acid ester of methoxypolyethylene glycol of mol.wt 1900 byan equal weight of the methacrylic acid ester of methoxypolyethyleneglycol of mol.wt. 4500. The modified latex obtained had a particle sizeof 0.67 micron and a solids content of 37%.

The modified latex (100 parts) was then diluted with methanol (50 parts)and transferred to toluene (350 parts) as described in Example 1(3).Methanol, water and toluene azeotrope were removed until thedistillation temperature was 110° C. The resulting dispersion ofmicroparticles in toluene had a viscosity of 1.2 poise and a solidscontent of 12.4%.

We claim:
 1. A process for the production of a stable dispersion in anon-aqueous liquid medium of composite polymer microparticles having adiameter of from 0.1 to 10 microns, each of which comprises a core ofcrosslinked polymer surrounded by and grafted to a layer ofnon-crosslinked polymer, the process comprising the steps of (1)polymerising one or more ethylenically unsaturated monomers, includingat least one crosslinking monomer, from which the crosslinked corepolymer of the microparticles is to be derived, in an aqueous medium ashereinbefore defined at a temperature at least 10° C. higher than theglass transition temperature of the core polymer in the presence in theaqueous medium as steric stabiliser of a block or graft copolymer whichcontains in the molecule a polymeric component which is solvatable bythe aqueous medium and another polymeric component which is notsolvatable by the aqueous medium and is capable of becoming associatedwith the polymer particles formed, the concentration of free monomer inthe polymerisation mixture being maintained throughout this process stepat a level such that at no time does the free monomer form a separatephase and the total amount of monomer polymerised being such that theresulting dispersion contains at least 20% by weight of microparticles,(2) polymerising, in the dispersion obtained from step (1), one or morefurther ethylenically unsaturated monomers, not including anycrosslinking monomer, from which the non-crosslinked polymer of theouter layer is to be derived, optionally in the presence of additionalblock or graft copolymer stabiliser, the concentration of such furthermonomer in the free state in the polymerisation mixture being maintainedat a level such that at no time does that free monomer form a separatephase, and (3) transferring the polymer microparticles from theresulting dispersion into a non-aqueous liquid medium which is a solventfor the non-crosslinkable polymer under such conditions that theparticles become stably dispersed therein.
 2. A process as claimed inclaim 1, wherein the ethylenically unsaturated monomer is selected fromacrylic acid, methacrylic acid and the alkyl esters thereof.
 3. Aprocess as claimed in claim 1 or claim 2, wherein the crosslinkingmonomer is a monomer which is polyfunctional with respect to thepolymerisation reaction.
 4. A process as claimed in any one of claims 1to 3, wherein the temperature of polymerisation is at least 30° C.higher than the polymerisation glass transition temperature.
 5. Aprocess as claimed in any one of claims 1 to 4, wherein thewater-miscible constituent of the aqueous medium is methanol or ethanol.6. A process as claimed in any one of claims 1 to 5, wherein the blockor graft copolymer stabiliser is produced in situ during thepolymerisation step (1) by introducing into the polymerisation mixture,before polymerisation begins, a precursor compound comprising in itsmolecule a polymeric component which is solvatable by the aqueous mediumand an unsaturated grouping which is capable of copolymerising with themonomer or monomers.
 7. A process as claimed in claim 6, wherein thesolvatable polymeric component of the precursor compound is derived froma polyethylene glycol, or a monoalkyl ether thereof, having a molecularweight in the range 2000-4000.
 8. A process as claimed in any one ofclaims 1 to 7, wherein the amount of further monomer polymerised in step(2) is at most 50% of the total weight of monomer used in steps (1) and(2) combined.
 9. A process as claimed in any one of claims 1 to 8,wherein one or more of the further monomers polymerised in step (2)carries, in addition to the polymerisable group, a chemically functionalgroup whereby the non-crosslinked polymer in the microparticles maybecome crosslinked by means of an external crosslinking agent afterapplication of the composition to a substrate.
 10. A process as claimedin any one of claims 1 to 9, wherein step (3) is accomplished byblending the aqueous dispersion obtained from step (2) with thenon-aqueous medium and then removing the water and the water-misciblesecond constituent of the aqueous medium by distillation as anazeotropic mixture.